The present invention relates to a motor apparatus for driving a motor at a fixed torque.
A smooth rotation with little torque change is required for motors used for tape transportation of tape recorders. The reason for this is that the torque change, or torque ripple, causes wow/flutter. One of the suitable methods to reduce the wow/flutter is to use a flywheel with a large inertia mass. The use of a large flywheel, however, prevents reduction of the size and weight of a tape recorder, makes slow the response of start/stop operation, and increases the products cost. For this reason, there is a great demand to satisfy a desired wow/flutter characteristic using a small-sized flywheel or, if possible, not using a flywheel. The best way is to reduce with torque ripple of the motor as low as possible, or to make the torque of the motor constant.
The requirement of torque ripple reduction is not necessarily applied only to a motor actuated with a constant rotation speed such as a capstan motor. Another motor requiring the reduction of the torque ripple is a disc rotation drive apparatus in which the rotating speed is faster as it is closer to the inner periphery of the disc so that its tangential velocity is made constant. Such an apparatus is applied to an optical disc system for recording and reproducing digital signals, which has recently been put into practice. In order to obtain a smooth rotation in the system, to torque of the motor used must be constant irrespective of the rotation speed being varied. In addition to the above system, use of the torque rippleless motor is frequently desired in an apparatus with a plurality of constant speeds. So far as the applicant knows, no torque rippleless motor applicable for actual products is available, except some specially designed motors.